sole plate design 2

[PDOTbridge]

I am attempting to determine the required thickness for a sole plate to be used at the pier bearing for a 2 span continuous steel multi-girder bridge (177’-177’). We have designed a circular laminated elastomeric bearing pad that will work at the location. Typically we do not do sole plate designs. We simply use a 1” thickness at the centerline of bearing, and bevel as necessary. However, due to the high loading at the pier bearing in this case, it seemed appropriate to check that the sole plate thickness was adequate. But I have not been able to find any design guidance/methodology in AASHTO or in our Design Manual. The best I have been able to do is a method given in the AISC Manual Of Steel Contruction, Volume II (LRFD, 2nd Edition) on pg. 11-50 for finding the minimum thickness of bearing plates.

Is there a method typically used for the design of sole plates? And where might I find it? Thanks for your help.

 

[mike80]

Ohio DOT has a formula for sole plates for rectangular elastomeric bearings: T = C(RW/(L+1))^.5 - beam flange thickness

where
T = sole plate thickness
C = .194 for grade 36 steel
= .167 for grade 50 steel
R = total reaction in kips including impact
W = width of elastomeric pad transverse to centerline of beam
L = length of elastomeric pad parallel to centerline of beam

They recommend using 80% of the thickness where bearing stiffeners are used, but require a minimum thickness of 1 1/2" for loads up to 200 kips and minimum of 2" above.

Hope this helps

Mike

 

[TTK]

I have never seen the Ohio DOT formula before, but sole plate thickness is usually governed by the greater of two conditions:

1. Allowable plate bending stress for overhang (from edge of flange to edge of pad).

2. Minimum thickness for load transfer and/or heat transfer during field welding (varies from owner to owner).

New York State DOT has a good example of #1 in their BDM which can be found at the following link:

http://www.dot.state.ny.us/structures/manuals/brman/sect12.pdf

Many States are moving towards 1.5" minimum sole plate thicknesses when field welds are used to connect the bottom flange to the sole plate directly above an elastomeric pad. Heat crayons can be used to minimize melting of the pad during welding, but as I said, the trend is too just specify a thicker plate and not worry about melting the pad.

I am curious to know how the Ohio DOT formula was developed, since it does not seem to factor in the actual overhang? It may assume their standard details are used so I would be careful on using that formula indiscriminately.

 

[PDOTbridge]

TTK: The formula shown in the link you provided is almost exactly the formula shown for determining the thickness of bearing plates indicated in the Manual Of Steel Construction Ninth Edition (Allowable Stress Design). The main difference being the length "free to bend" considered. It would seem that the one formula was based on or derived from the other. Do you know if this is true?

 

[TTK]

I don't have the steel manual in front of me but the formula in the link I provided is just a simple cantilever beam theory.

The plate is assumed to be fixed at the edge of the flange (or bearing pad if it is smaller than the flange) and the moment in the plate is calculated assuming a uniformly distributed load on a cantilever beam.

The square root comes into the formula because the section modulus of the beam (plate) is cubic, and it cancels out to a squared term in the process.

Try deriving it yourself by solving for the stress in the beam (plate) but leave the thickness "t" as a variable.

Good Luck!